Tracking systems have numerous applications, including those detailed in the following patents; autonomous vehicles (U.S. Pat. No. 6,535,114), monitoring systems (U.S. Pat. No. 6,690,374), sports cameras (U.S. Pat. No. 6,567,038), conference video systems (U.S. Pat. No. 6,507,366), surgery (U.S. Pat. Nos. 6,725,079 and 6,662,036), positioning (U.S. Pat. No. 6,490,473), inspection (U.S. Pat. No. 6,259,960), spotlighting (U.S. Pat. No. 6,079,862), and machining (U.S. Pat. No. 6,429,404). Tracking systems use certain distinguishing features of a target being tracked as sources for detecting, tracking, or engaging the target. These features can be any form of energy emitted or reflected from the target including radio, millimeter-wave, infrared, visible light, laser, ultraviolet, and sound. Each energy source has different characteristics in terms of accuracy, response speed, maximum operation range, and operation environment such as weather and day/night. Since each electro-magnetic or mechanical wave has different characteristics, disadvantages of one wave can be compensated by advantages of others when some of them are employed together. For example, a tracking system can use both infrared and millimeter wave for providing all weather operation capability. Also, a tracking system can use both infrared and visual light for providing a day/night operation capability.
Many applications using tracking systems require high positioning accuracy and fast tracking ability with a reliable structural stability; e.g. star trackers, optical seekers, and robot vision. Optical tracking devices are well suited for these purposes. The optical tracking device typically requires an imaging system comprising a lens unit, an imaging unit having image sensors, and an image processing unit. For the image sensors, various solid-state focal plane arrays for different energy sources can be used such as CCD (charge-coupled device) or CMOS APS (complementary metal-oxide-semiconductor Active Pixel Sensor) for visible light and QWIP (Quantum Well Infrared Photodetector) for infrared. The CCD image sensor used to be a choice for many optical tracking devices for a long time because of its high sensitivity and high signal to noise ratio. However, it comes with high cost and slow response due to pixel to pixel charge transfer and tends to result in a large package because the control circuitry cannot be integrated into an image sensor. Currently, the CMOS APS has shown nearly equivalent to or better performance than the CCD image sensor. In addition, it provides many advantages including on-chip circuit integration, random pixel access, and low power consumption.
The imaging systems for the conventional optical tracking devices can be divided into two groups. One group uses a single camera while the other group uses a plurality of cameras. The imaging system with a single camera generally has a simpler configuration and image processing scheme than its counterpart. However, the optical tracking device with the single camera cannot produce three-dimensional image information. Furthermore, it is difficult to extract distance information from the single camera unless it is used in combination with a device such as a range finder. Additionally, it is easy to lose track of fast moving targets because a field of view (FOV) of the single camera is limited. Therefore, the imaging system with the single camera typically has a high ‘tracking dropout’ rate.
The imaging system with the plurality of cameras is capable of generating three-dimensional image information by disposing the lenses of cameras to have different viewing angles in order to use binocular parallax phenomena. Also, it can be configured to reduce the tracking dropout rate by using at least two lenses with different fields of view; for example, one narrow FOV for high resolution image and one wide FOV for low tracking dropout rate. U.S. Pat. No. 6,734,911 to Lyons describes such a system using a dual-angle lens to obtain both wide-angle image and narrow-angle image of a scene. However, the system described in the '911 patent uses a very complex lens configuration and generates large image distortion for wide-angle image. Furthermore, this system requires that the attitude of the imaging camera be adjusted by a servo motor. The imaging system with multiple cameras yields a complicated structure and requires a complicated image processing scheme.
Optical tracking devices usually require a camera attitude control system so that the image of the target can be in the center of the image sensor. Typically, the attitude of the camera is adjusted by using a servo motor or a gimbal system having macroscopic mechanical movements as disclosed in the U.S. Pat. No. 6,507,366 to Lee. These optical tracking devices with macroscopic mechanical movements, however, tend to be bulky and heavy with low imaging speed and high power consumption and involved with complicated alignment and calibration processes.
FIGS. 1 (1a˜1b) are block diagrams of conventional optical tracking devices. In FIG. 1a, a conventional optical tracking device 11 includes an imaging system 12 configured to capture images. The imaging system 12 may use either a visual or infrared wavelength. The imaging system using a visual wavelength may acquire a color image, but do not perform well at night or in heavy fog. The imaging system using an infrared wavelength cannot acquire a color image, but may be used at night or in heavy fog. Regardless of whether the imaging camera uses visual or infrared light, the construction of the imaging system may be similar in its components.
The imaging system 12 comprises an image sensor 13, configured to sense an image. The image sensor 13 may sense either a visual or infrared wavelength. The optical tracking device 11 also includes an image processor 14, communicatively coupled to the image sensor 13, configured to process the images sensed by the image sensor 13 and to generate an output signal 15. The image processor 14 may include a target-identifying algorithm.
In FIG. 1b, a conventional optical tracking device 16 includes a tracking controller 17, communicatively coupled to the image processor 14. The image processor 14 is configured to generate image data 18A and position information of a target and the tracking controller 17 is configured to generate a tracking signal 18B. In one embodiment, the tracking controller 17 includes a camera attitude control algorithm and attitude controller. A movement system 19 is mechanically coupled to the imaging system 12 and communicatively coupled to the tracking controller 17, configured to adjust the attitude of the imaging system 12 in response to the tracking signal 18B from the tracking controller 17. In one embodiment, the movement system 19 may include, for example, a servo or gimbal system.
The advanced optical tracking device have to track a fast moving target, provide the three-dimensional image information of the target, and compensate the aberration of the optical tracking device with simplified construction.